Optoelectronic storage media such as compact discs (CDs) and digital versatile discs (DVDs) are commonly used for storing digital data. A typical disc is a little over one millimeter thick and is predominantly made out of a material such as plastic. Data is coded onto the disc by creating a series of bumps in the plastic. The plastic can then be covered by a reflective material such as aluminum and a protective material such as acrylic. The series of bumps on the disc form a data track that begins at the center of the disc and spirals outward. In a CD, for example, the bumps may be smaller and may be spaced differently than in a DVD, high-density DVD (HD DVD), or Blu-ray Disc.
FIGS. 1a and 1b show a typical laser 101 and optical detection unit 102 in one type of optoelectronic processing device. The laser 101 directs a beam of light 103 towards a disc 104. If the beam of light 103 strikes a bump 105 as in FIG. 1a, then it might be reflected towards the optical detection unit (ODU) 102. If the beam of light 104 strikes where there is not a bump, sometimes referred to as a land 106, as in FIG. 1b, then the beam might be reflected away from the ODU 102. The changes in reflection can be transmitted as bits of digital data by the ODU 102 to processing circuitry 107 which processes the received data and provides an appropriate output, depending for example on whether the output is to be music, video, or another type of data.
DVDs, HD DVDs, China Format HD Discs (C-HD), Blu-ray Discs, and other optical disc-reading systems work using the same principles as the CD system described above, but they utilize lasers of a smaller wavelength than CDs that allow the bumps and lands to be smaller and spaced more closely together, thus allowing for more data to be stored on a disc of the same physical size. FIG. 2 shows an example of an optical disc 210 that may be used in such a system. The disc 210 contains a hole 211 at the center so that a drive motor can rotate it in the direction shown by the arrow 212. As the disc 210 rotates, a head assembly 213 containing an optical detector focuses on the reflection of a laser from the disc surface 210. In order to properly focus the optical detector, the disc-reading device must move the head assembly depending on the location of the data track being read. Because the bumps on the disc surface 210 are so small and must be read in rapid succession, the head assembly 213 must be able to move with extreme precision and achieve focus rapidly. Accordingly, the head assembly 213 is configured to move in two different manners. First, it can slide on an arm 214 along the x-axis, and second, it can make minor adjustments to its focus by rotating along the y-axis, thus changing the angle of the optical detector relative to the x-axis. This angle is commonly referred to as the radial tilt.
It is also common for the head assembly 213 to include a means, such as a drive motor, for rotating about the x-axis in order to adjust for a tilt in the disc 210 or a tilt of the optical detector. The angle of the optical detector relative to the y-axis is commonly referred to as the tangential tilt. In a typical system, both radial tilt and tangential tilt may be present.
Several methods exist in the art for calibrating the radial and tangential tilt. One such method includes sweeping a range of angles to determine which angle produces the fewest errors. Another method includes making incremental changes to the tilt based on whether signal quality, as judged by a signal characteristic such as signal-to-noise ratio, is improving or worsening. Both these methods, however, are undesirable because they are slow to arrive at a desired tilt angle. A more sophisticated method can be implemented that measures a signal-to-noise-ratio over separate and independent portions of a signal and uses the difference between those portions to calculate a desired tilt. Such a method may be faster than other methods known in the art, but it is still undesirable because it requires monitoring multiple independent variables, and as a result can require multiple data channels when configured into hardware, thus increasing the complexity and cost of implementation.
Therefore, there exists in the art a need for a new method and system for accurately and rapidly calibrating the radial and/or tangential tilt of an optical detection unit in an optical disc reading system. Additionally, there exists in the art a need for such a system that can be easily and inexpensively implemented into the hardware of an optical disc reading system.